If you are using videos in your portfolio, both the video and any image attachments will appear on the post. You can hide the featured image in a variety of ways, but the following is the quickest and lowest-maintenance solution.
This is no longer an issue in Gigawatt 2.0 and later. Please ensure your theme is up to date. For help upgrading to Gigawatt 2.0 from 1.8.3 or earlier, see Gigawatt 2.0